Projects Synthesis Materials Fabrication Devices Characterization Theory
Magnetic   Photoluminescence  
X-Ray Diffraction  
Magnetic
Since winter 2005 we have an Adiabatic Demagnetization Refrigerator (ADR). This system was developed together with Cavendish Laboratory at Cambridge University to suit our special purposes and supported by a CNNC grant.

The cooling process is based on the paramagnetic properties of a certain salt. In the beginning of the process the salt pill is in very good thermal contact with a heat sink. If one applies a magnetic field one will align the spins of the system and thereby lower the entropy of the spin system. This process is isothermal due to the very good thermal link to the heat sink. If one now decouples the salt pill from the thermal bath and subsequently decreases the magnetic field the spins will un-align which is equivalent to an increase in entropy dS of the spin system. Therefore the spin system has to absorb an amount dQ1 = T dS of heat. Since the system of the salt pill and any experimental stage mounted on it is decoupled from the thermal bath and cannot exchange any heat with its environment (i.e. dQtot= 0), this heat dQ1 causes a cooling of the system according to dQ2 = C dT so that dQtot = dQ1 + dQ2 = 0, where C is the system's specific heat. This effect is at the heart of the refrigeration process and was first proposed by W.F. Giauque and independently P.J.W. Debye in 1927. In the case of the ADR in our lab the demagnetization process is started from a pre-cooled 1.6 K stage. This enables us to finally reach base temperatures of 50 mK. Due the unique properties of our system it is possible to continuously sweep the temperature over nearly three decades from 50 mK up to 40 K. This feature is essential to investigate changes in the power-law dependence of various physical properties.
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Photoluminescence
Photoluminescence (PL) spectroscopy is a contactless, nondestructive method of probing the electronic structure of materials. Light is directed onto a sample, where it is absorbed and imparts excess energy into the material in a process called photo-excitation. One way this excess energy can be dissipated by the sample is through the emission of light, or luminescence. In the case of photo-excitation, this luminescence is called photoluminescence. The intensity and spectral content of this photoluminescence is a direct measure of various important material properties.

Photo-excitation causes electrons within the material to move into permissible excited states. When these electrons return to their equilibrium states, the excess energy is released and may include the emission of light (a radiative process) or may not (a nonradiative process).

The followign are the general applications of PL Band Gap Determination: The most common radiative transition in semiconductors is between states in the conduction and valence bands, with the energy difference being known as the band gap. Band gap determination is particularly useful when working with new compound semiconductors.

Impurity Levels and Defect Detection: Radiative transitions in semiconductors also involve localized defect levels. The photoluminescence energy associated with these levels can be used to identify specific defects, and the amount of photoluminescence can be used to determine their concentration.

Recombination Mechanisms: The return to equilibrium, also known as "recombination," can involve both radiative and nonradiative processes. The amount of photoluminescence and its dependence on the level of photo-excitation and temperature are directly related to the dominant recombination process. Analysis of photoluminescence helps to understand the underlying physics of the recombination mechanism.

Material Quality: In general, nonradiative processes are associated with localized defect levels, whose presence is detrimental to material quality and subsequent device performance. Thus, material quality can be measured by quantifying the amount of radiative recombination.
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X-Ray Diffraction
X-Ray Diffraction (XRD) is used to probe the crystal structure of micro and nanostructures, thin films and bulk samples in a nondestructive way. The principle of XRD is Bragg diffraction which is defined by nλ= 2dsinθ=2Δ, where n is an integer, λ is wavelength, d is the inter-planar spacing in the crystal lattice, θ is the angle between the sample surface and incident beam and Δ represents the path difference. As might be guessed from the formula, this technique is based on the interaction of the incoming x-ray beam with the crystallographic planes in the sample.


In the figure above, the blue lines represent planes of atoms in the reciprocal lattice where dhkl is the distance between planes defined by the Miller indices h, k and l. In nanoscience, either a powder or thin film sample is prepared and loaded into the system where it is rotated through a chosen angle, as can be seen in the figure. The incident x-ray beam is reflected out of the sample and may go through several slits (collimator and filter) before entering into a detector which registers the intensity at an angle with respect to the sample plane.

When an incident beam angle is equal to the angle of reflection, the beams reflected are in phase with each other; that is they have a path difference with an integer number of wavelengths. This condition is referred to as the Bragg diffraction condition. At certain characteristic angles for each material the Bragg diffraction condition will be met and large intensities of the reflected beam will be detected whereas at most angles small or no intensity of the reflected beam will be detected. These intensities are seen as peaks in a plot of vs. Intensity. The peak positions, peak shapes and relative intensities are directly correlated to crystallographic spacing, phase, and grain or particle size. Thus XRD spectra can be used to determine various material parameters.

We analyze micro and nanoscale particles like nanowires and quantum dots as well as thin films for electron/ion beam lithography resist development. The species of a material can be identified, its crystal structure and phase; in some cases we can also estimate grain size or nanoparticle size via Scherrer broadening. XRD is a fundamental technique and is complementary to other techniques, such as SEM. Nanoparticle shape or morphology can not be assessed by XRD, but XRD can identify the crystallinity (amorphous, polycrystalline, mixed-phase, single crystal) whereas SEM cannot.
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